Laser scanning has been achieved by the mechanical motion of a prism or mirror on which a coherent optical beam is incident and by the use of acousto-optic crystals which utilize ultrasonic wave propagation to diffract a coherent optical beam. The mechanical or propagating acoustic wave nature of the prior art limits the scanning speed of such devices, or adversely affects their reliability or economic utilization.
More recently, optical scanners have been proposed which utilize the movement of an interference pattern across an image or target trace. In U.S. Pat. No. 3,626,321 a moving interference pattern is achieved by a system that utilizes a plurality of discrete gas lasers to produce a plurality of coherent light beams with ordered geometrical intervals and frequency differentials from each other, the phased relationships between the beams providing the moving interference pattern. In U.S. Pat. No. 3,691,483, a phase-locking laser, an array of variable phase shifters, an array of semiconductor lasers and a control computer are utilized to provide a moving interference pattern for optical scanning. The moving interference fringe scanning systems alluded to require precise alignments, spacings and frequency control of a plurality of discrete components and require phase delay elements outside of the laser cavity.
The problems encountered with discrete component systems are not present in the solid state scanning system of U.S. Pat. No. 3,701,044 to Paoli et al. In the Paoli et al system a plurality of parallel strip electrical contacts are provided on one surface of a rectifying junction diode, the contacts being so spaced that the optical fields produced by adjacent lasing areas are optically coupled to produce a locked oscillation, that is, a composite beam. Paoli et al states that by introducing phase shifts between adjacent optically coupled lasing areas, the direction of the composite beam is changed to produce scanning. The Paoli et al system does not teach decoupling of the composite beam produced by adjacent optically coupled lasers thereof and hence cannot provide relative phase shifting of different optical fields.